In a Wireless Local Area Network (WLAN) an Access Point (AP) can communicate with each of a plurality of member stations, over a communications channel. Each member station may be, for example, a cellular telephone, a portable computer, or any other wireless receiver and transmitter unit.
As defined by IEEE 802.11 WLAN standards, there are two main communications-channel access-methods by which a member station communicates with an Access Point (AP) in a Wireless Local Area Network (WLAN). One of these methods is scheduled access and the other is contention-based access. In a scheduled access approach, an Access Point (AP) polls each of its member stations and each member station can only transmit over a communications channel after being polled. Since only one member station communicates over a communications channel at a time, there can be no conflict between member stations, and therefore contention resolution algorithms are not needed. Thus in a scheduled access method, the overall efficiency is high and the Quality of Service (QoS) that each member station receives is guaranteed by a polling schedule. Unfortunately, the majority of WLAN hardware available today does not support scheduled access.
The majority of WLAN hardware today implement a contention-based method, in particular the Carrier Sense Multiple Access Collision Avoidance (CSMA-CA) method. In this contention based approach, a member station is not polled, but rather when the member station wants to transmit to the Access Point over the communications channel, it first senses whether the channel is available. If the channel is determined to be busy, the particular member station will wait for a waiting time period before attempting to transmit again. The waiting time period includes an Arbitration Inter-Frame Space (AIFS) followed by a Contention Window (CW). Only if the communications channel remains idle during the whole waiting time period may the particular member station begin its transmission over the communications channel to the Access Point.
The use of a waiting time period in a contention based approach reduces the possibility of transmission collisions: two member stations transmitting over a communications channel at the same time. However, the waiting time period results in delays in packet transmission from the member stations to the Access Point (AP).
The waiting time period in the contention based approach of CSMA-CA is determined by several parameters. The Arbitration Inter-Frame Space (AIFS) is determined by a configuration parameter called Arbitration Inter-Frame Space Number (AIFSN), which specifies the length of the AIFS in terms of time slots. The Contention Window (CW) for a particular transmission is determined randomly between 0 and an upper bound. The initial upper bound is a parameter called minimum CW, or CWmin. Every time a transmission fails, the upper bound is doubled until it reaches the maximum upper bound, which is the called CWmax.
The parameters AIFSN, CWmin, and CWmax have been set differently for different Quality of Service (QoS) classes. Setting theses parameters lower results in shorter wait times and a greater likelihood of access (i.e. a higher access priority) to a communications channel. For example, a voice data quality of service class might have lower AIFSN, CWmin, and CWmax values than a video quality of service class, resulting in a greater likelihood of transmission of voice data then video data over a communications channel. Setting these channel access parameters differently for different data traffic or communication classes is known as Enhanced Distributed Channel Access (EDCA) and the parameters AIFSN, CWmin, and CWmax can be called EDCA parameters.
EDCA offers no quality of service (QoS) guarantee to individual traffic streams. It only offers statistically higher channel access probability to communication traffic of higher Quality of service (QoS) classes. Traffic streams within the same service class still rely on contention based resolution methods to compete for channel access. Channel access is not efficient because of the time spent resolving contentions. As the number of traffic streams increases, such as with an increase in the number of member stations in a Wireless Local Area Network (WLAN), the number of traffic streams belonging to the same class will increase resulting in more contentions. In addition, since traffic streams of the same Quality of Service (QoS) class are treated the same, they have the same opportunity of transmission, which may not be desirable.
U.S. published patent application no. 2006/0187840 A1 to Cuffaro et al., discloses a method of modifying contention windows and Arbitration Inter-Frame Space (AIFS) for low priority traffic when congestion is detected on a wireless medium. A contention window associated with low priority traffic is incremented and a processor updates an EDCA parameter set included in frames transmitted by a transmitter. The size of the AIFS can be increased in parallel with the increase in the contention window.